Related-art image forming apparatuses such as copiers, printers, facsimile machines, and multifunctional machines including at least two of these functions generally employ a pair of registration rollers that serves as a sheet correction unit to correct skew of a sheet of paper before an image formed on an image carrier is transferred onto the sheet of paper.
FIG. 1 illustrates a schematic configuration of a related-art sheet feeder including a pair of registration rollers therein.
In the configuration of the related-art sheet feeder, here employing a pair of registration rollers 300 as illustrated in FIG. 1, a leading edge of a sheet of paper S conveyed by a sheet feed roller 200 in a direction indicated by arrow “Y” in FIG. 1 is abutted against a nip contact area of the pair of registration rollers 300, which remains stopped, thereby deflecting the sheet S a given amount and generating a restorative force. Then, the restorative force causes the leading edge of the sheet S to be aligned along the nip contact area, thereby correcting skew of the sheet S. Thereafter, in synchronization with transport of an image is transferred onto an image transfer member, the sheet feed roller 200 and the pair of registration rollers 300 are driven to convey the sheet S further forward. The above-described operation is generally used to achieve an uncomplicated and low-cost configuration. A drawback of this arrangement, however, is that when the sheet feed roller 200 is stopped after the sheet S has been deflected, impact noise is generated.
As illustrated in FIG. 1, a sheet separation member 500 is provided at the position facing the sheet feed roller 200 to separate the sheet S one by one. The sheet separation member 500 is supported or held by a holder 600 and biased by a biasing member 700 toward the sheet feed roller 200. While the movement of the holder 600 in a direction parallel to a sheet conveyance direction is restricted by a positioning member 800, the movement in a direction toward an axial center of the sheet feed roller 200, that is, in a direction substantially perpendicular to the sheet conveyance direction is free and not regulated. A more detailed description of this arrangement follows.
FIG. 2 is an enlarged cross-sectional view illustrating the positioning member 800 and components around the positioning member 800.
As illustrated in FIG. 2, the positioning member 800 is inserted into and slidably move along a groove 900 provided in a sheet feed tray 400. With this configuration, the positioning member 800 is allowed to move in a direction substantially perpendicular to the sheet conveyance direction (as indicated by arrow “X” in FIG. 2) but is restricted from moving in a direction parallel to the sheet conveyance direction (as indicated by arrow “Y” in FIG. 2). Strictly speaking, however, in order to assure the slidable movement of the positioning member 800 both in the sheet conveyance direction and in the direction substantially perpendicular to the sheet conveyance direction, a small gap G is provided between the positioning member 800 and the walls of the groove 900 to form clearance in the sheet conveyance direction.
If such space is formed between the positioning member 800 and the groove 900, when the driving of the sheet feed roller 200 is stopped with the sheet S deflected, the sheet feed roller 200 rotates a slight amount in reverse in response to the restorative force F caused by the deflection of the sheet S described above. Due to this action, the holder 600 receives a force that is exerted to an upward side in the sheet conveyance direction. Then, the positioning member 800 abuts against the groove 900, which causes impact noise.
In order to avoid the impact noise caused by the abutment of the positioning member 800 against the groove 900, the sheet feed roller 200 and the pair of registration rollers 300 can have separate drive sources so that the sheet feed roller 200 can be prevented from rotating in reverse. However, the addition of another drive source increases the size and cost of the image forming apparatus, and therefore the sheet feeder cannot be implemented in a low-cost image forming apparatus.
In order to avoid the above-described problem without adding another drive source, Japanese Patent Application Publication No. 04-133070-A proposes an arrangement in which the driving of the sheet feed roller is temporarily stopped before the leading edge of a sheet conveyed by the sheet feed roller is abutted against the pair of registration rollers, after which the sheet feed roller is again driven. There, the leading edge of the sheet is deflected, and at the same time the pair of registration rollers is driven to rotate with the sheet feed roller being rotated so as to convey the sheet. Thus, by rotating the sheet feed roller continuously without stopping after the deflection thereof is formed, the reverse rotation of the sheet feed roller due to the restorative force generated by the deflection of the sheet can be avoided, thereby preventing occurrence of impact noise caused by the movement of the positioning member.
Although the method described in Japanese Patent Application Publication No. 04-133070-A can eliminate the impact noise caused by the positioning member after the deflection of the sheet is formed, another problem can arise because of the above-described solution. In a related-art control of skew prevention, the driving of the sheet feed roller is temporarily stopped with the sheet remaining in contact with the pair of registration rollers, and therefore the leading edge of the sheet is aligned accurately. By contrast, in the method disclosed in Japanese Patent Application Publication No. 04-0133070-A, the driving of the sheet feed roller is stopped before the sheet contacts the pair of registration rollers. Therefore, due to rigidity of the sheet and variation of the coefficient of friction, adhesion of paper dust to the sheet feed roller or a separation member, aging of the sheet feed roller and the separation member and the like, slippage occurring when the sheet is fed from the sheet tray causes the position where the sheet is stopped before the pair of registration rollers to vary.
For example, when the leading edge of the sheet is stopped at an upstream side from a predetermined position in a sheet conveyance direction due to slipping of the sheet, it is likely that the pair of registration rollers starts driving before the sheet contacts the pair of registration rollers. This can fail in deflecting the sheet, which can vary the position of the leading edge of the sheet and/or cause accurate skew correction to fail.
In order to prevent failure in deflecting the sheet, a greater deflection can be given to the sheet that takes such slippage into consideration. However, since greater deflection generates greater restorative force, if there is no space for the deflection of the sheet in a sheet conveyance pathway, it is likely that the leading edge of the sheet exceeds a nip contact area formed by the pair of registration rollers, again generating variation in the position of the leading edge of the sheet.